That sounds reasonable. The Maxxus has 3 pumps where the other Sundance spas in question have only 2. And for those spas, the 60 amp setting is necessary to run 2 pumps and the heater all at once. Suppose there isn't enough headroom to get that third pump going and the heater on the same 60 amp setting. And going up any more in power would certainly add a LOT to the cost of wiring the spa.
6 gauge copper would only be good for 60 amps, you'ld likely have to bump up to 4 or 2 gauge- and you do not want to work with that stuff unless you have to. It's very stiff, and very expensive. You'd also need fairly large conduit as well.
With all 3 pumps on at once, I can't imagine your heat would drop all that much anyway, unless it was very cold out.